feat(soroban): implement repayment contract and verify unit tests

[Jess52487]

Description

Closes #12

This PR introduces the RepaymentContract (Soroban Smart Contract) written in Rust, along with its configuration files and unit tests.

Key Changes

• Smart Contract Implementation: Created contracts/repayment/src/lib.rs implementing assignment and repayment tracking logic for driver contracts, handling overpayment logic, and providing query endpoints.
• Security Check: Enforced authorization checks on admin/driver roles via require_auth().
• Cargo Workspace Setup: Configured Cargo.toml and .cargo/config.toml in the repository root.
• Warnings & Test Refactoring: Cleaned up compiler warnings and finalized the unit tests.
• Git Ignore configuration: Added /target to .gitignore.

Verification & Testing

All 6 unit tests pass successfully under the stable-x86_64-pc-windows-gnu toolchain:

• test_initialization_and_assignment
• test_repayments
• test_overpayment_behavior
• test_unauthorized_repayment_fails (should panic)
• test_unauthorized_assignment_fails (should panic)
• test_unauthorized_caller_rejected